Only 3.4 percent of girls of middle school age meet physical activity (PA) recommendations. As middle school girls progress from 6th- to 8th- grade, moderate to vigorous physical activity (MVPA) declines 4 percent per year, contributing to weight gain. The decline is particularly evident among urban, minority girls of low socioeconomic status (SES), supporting that increasing MVPA is a vital public health priority for this group. The 17-week Girls on the Move (GOTM) intervention applies the Health Promotion Model (HPM) and Self- Determination Theory (SDT) and has 3 components. Two individual-level components occurring during school hours include: (1) a 15-minute face-to-face motivational, individually tailored counseling session with a school nurse to support each girl's MVPA (0 wks.), and (2) an interactive Internet-based counseling session during which each girl receives motivational, individually tailored feedback messages (9 wks.). At both sessions, counseling reflects the motivational interviewing communication style and is individually tailored based on each girl's key responses gleaned from computerized questionnaires assessing thoughts and feelings about PA. A group-level component, 90-min. PA Club (offered 4 days/wk.), includes MVPA and provides an important venue after school for assisting girls to establish a behavioral pattern of MVPA. PA Club instructors lead the PA Club (e.g., qualified individuals from community; teachers, including physical education; and sports team coaches). To bridge the gap between the after-school PA Club and the counseling during school hours, the school nurse attends the PA Club two times per month to do MVPAs with the girls and offer encouragement. The aim of the 5-year school-based group randomized trial is to evaluate the efficacy of the GOTM intervention in increasing middle school girls' minutes of MVPA (primary outcome) and improving cardiovascular (CV) fitness, body mass index, and percent body fat (secondary outcomes), immediately post-intervention and at 9- month follow-up. Cognitive and affective mediating variables include: perceived benefits of PA (HPM), barriers to PA (HPM), PA self-efficacy (HPM), social support (HPM), enjoyment (HPM), and motivation (SDT), all of which are related to MVPA. In addition to addressing all mediating variables, each intervention component incorporates three basic needs proposed by SDT for promoting intrinsic motivation to drive behavior: competence, autonomy, and relatedness. Based on demographics of the schools, a low-SES, mixed-race, predominately African American, sample is expected. Eight urban schools will be randomly assigned to the PA intervention (n = 4) or comparison condition (n = 4) in the fall of yrs. 2, 3, and 4 (total N = 8 X 3 = 24 different schools). Sixty-two girls will be recruited in each school during each of the three years. The comparison condition will include two 90-minute health-promoting workshops. Our long-term goal is to increase MVPA as a means to address the high overweight and obesity prevalence among adolescent girls, thus reducing their risk for adverse CV health outcomes.